The technology for manufacturing circuits comprising components mounted on printed circuit boards has evolved continuously during the past fifty years keeping pace with the evolution of discrete semiconductor devices to present high density integrated circuits.
The current "surface mount technology" (SMT) is applied to printed circuit boards (PCB) that have a plurality of layers of wiring with contact points from each layer extending up to "pads" located on the top and bottom surfaces of the board. Components are soldered to the pads.
The process starts with a bare board referred to as a fab which has been manufactured with layered wiring and surface art comprising exposed pads. The fab is loaded into the printing machine utilizing a stencil to print solder paste on the pattern of pads on the fab. Then the fab is conveyed to a "pick-n-place" machine which places components in position on the fab. Then the fab is placed in a "reflow" oven where the solder is melted and then allowed to cool so that the components are now securely soldered to the fab.
A common requirement for all of these processes is that each fab must be supported as it is carried through each step of the process. This is generally accomplished in the industry by mounting the fab on a supporting pallet which is designed for a specific station and is unique to that specific fab.
The state of the art SMT pallets used in the reflow step have an opening framed by a shoulder for supporting the board over the opening. The tolerance between the step down wall of the shoulder and the board is typically 0.015" to allow for differential expansion between the pallet and board.
The double sided PCB has the problem that, in a reflow operation, components soldered to one side of the board during a first reflow step can fall off the board when the board is turned over for the second reflow step.